Float Adjustments
The Haystack SU Needle program is a DOS based collection of small utilities used to fine tune your needle selection. This process works best if you have access to an exhaust-gas analyzer (or exhaust temperature gauge) as you will need to know if/where your engine is running rich and/or lean. Using the Haystack programs, you can find needles with richer or leaner characteristics to suit your needs. Refer to the Haystack manual for further information.
I have used the ADD feature of Haystack to input the Datsun-supplied SU needles that Nestor Moya has measured (M39, M43, M66). The accuracy of the data is now high because of his work (when compared to the numbers published in the old Hot To book). The added Datsun data can be used as a guideline in your tuning efforts.
HAYSTACK (c) copyright 1990, 1991, 1992.
Another method of fine-tuning your SU carbs, especially with a modified motor, is changing the dome spring for a stiffer or softer spring, or a combination of both. Think along the lines of a progressive suspension spring. A softer spring will let the SU piston lift fast, providing more airflow at lower RPM. A stiffer spring will resist the upward movement of the piston, providing a degree of fuel enrichment for acceleration. A combination of the two would allow for more airflow at the low end while providing more careful fuel metering at higher RPMs. Your goal is to find the dome spring that will allow the piston to reach maximum lift only at peak horsepower. This is just another variable along the path of perfecting your SU setup. And you thought SUs were simple...
I have calculated the approximate spring rate for my dome springs. These measurements are based on a dome spring from a set of 38mm flat-top Hitachi-SUs. You may download the Excel spreadsheet if you wish to enter your own information.
| Spring Rate for 38mm Flat-Top Springs | ||
| Number of coils | 28.500 | |
| Coil diameter | 0.938 | |
| Wire O.D. | 0.035 | |
| Free length (in inches) | 4.000 | |
| lbs/in= | 0.096 | |
Rating a dome spring by pounds per inch seems a little hokey. British dome springs are rated in ounces, so multiply your result by 16 to get a comparable number. 0.096 x 16 = ~1.5 oz/in.
Typical British dome spring rates: This information was gained from MiniMania.
As you can see, the calculated rate for my dome springs is pretty low. I have a set of new Z dome springs (#16346-12210 @ $8 ea.), and they are identical to my 38mm springs. See the Dome Spring Experiment page to find out what's up with that.
Float AdjustmentsA critical, yet often overlooked area of SU carbs is the proper setting of the floats. This is a very important step! Setting the float level so that you don't run out of gas while driving will get you from point A to point B, but if you're going to do it, why not do it right?
Well, because it can be a pain in the butt. However, if your float levels are not set correctly, the level of fuel in the nozzles will not be correct either, exposing more or less of the needle than is optimum (more info). If you have visited the SU links above, you will have read a few different methods for setting the floats. I recommend using a float gauge (if you have one) or some method of visual inspection of the float level: